NIST Center for
Neutron Research
National
Institute of Standards and Technology Gaithersburg, MD 20899-6102

Wei Zhou is a materials chemist at
NCNR/NIST. His research
interests are in the areas of novel porous materials,
computational materials design, neutron diffraction and spectroscopy.
His recent work focuses on developing advanced materials
for gas adsorption, storage, and separation (CH4, H2,
CO2, C2H2 etc.), by combining various experimental
techniques and first-principles calculations. The
materials under investigations include porous metal-organic
frameworks (MOFs),
hydrides etc. Shown below are some
highlights of his recent work.

06/2013 Unusual and highly tunable
missing-linker defects found in Zr-MOF UiO-66UiO-66 is a highly important
prototypical Zr-MOF. Using high-resolution neutron power
diffraction technique, we found the first direct
structural evidence showing that real UiO-66 material
contains significant amount of missing-linker defects,
an unusual phenomenon for MOFs. We show that by varying
the concentration of the acetic acid modulator and the
synthesis time, the linker vacancies can be tuned
systematically, leading to dramatically enhanced
porosity. Comparing the gas adsorption of hydroxylated
and dehydroxylated UiO-66, we found that the former
performs systematically better than the latter,
suggesting the beneficial effect of the −OH groups. The
preferred gas adsorption on the metal center was
confirmed by neutron diffraction measurements, and the
gas binding strength enhancement by the −OH group was
further supported by our first-principles calculations.
H. Wu, Y. S. Chua, V. Krungleviciute, M. Tyagi, P. Chen,
T. Yildirim, W. Zhou*,
J. Am. Chem. Soc., 135, 10525–10532
(2013).

03/2013 Exceptional mechanical stability
of Zr-MOFs and its origin revealedMOFs with high porosity usually
exhibit weak mechanical stabilities, in particular,
rather low stabilities against shear stress. This
limitation remains one of the bottlenecks for certain
applications of porous MOFs. We found that UiO-66
exhibits unusually high shear stability. Its minimal
shear modulus is an order of magnitude higher than those
of other benchmark highly porous MOFs, approaching that
of zeolites. Our analysis clearly shows that the
exceptional mechanical stability of UiO-66 is due to its
high framework connections. Our work thus provides
important guidelines for developing new porous MOFs
targeting at high mechanical stabilities.
H. Wu, T. Yildirim, and W. Zhou,*
J. Phys. Chem.
Lett., 4, 925–930
(2013).

10/2010
Structural stability and elastic properties of
prototypical COFs studiedWe report the first
investigation of the structural stabilities and elastic
properties of covalent organic frameworks (COFs), a
relatively new class of porous crystalline materials.
Representative 2D COFs were found to prefer shifted AA
stacking, somewhat similar to graphite. The shear moduli
of 2D COFs are exceedingly small, suggesting that the
layer–layer coupling in 2D COFs is rather weak, and
stacking faults may widely exist. Representative 3D COFs
were found to exhibit relatively low elastic stiffness
overall. In particular, COF-108, the least dense crystal
known, exhibits rather low bulk and shear moduli. Our
findings provide important structural and physical
details to be considered in the further development of
COF materials. W. Zhou,*
H. Wu, and T. Yildirim, Chem. Phys. Lett., 499, 103–107
(2010).

03/2009
Open metal sites found to play an important role for CH4
storage in MOFsWe found that MOF compounds M2(dhtp) (also known as MOF-74
analogues) possess exceptionally large densities of open
metal sites. By adsorbing one CH4 molecule per
open metal, these sites alone can generate very large
methane storage capacities, approaching the DOE target for
material-based methane storage at room temperature. Our adsorption isotherm
measurements at 298 K and 35 bar for the M2(dhtp)
compounds yield excess methane adsorption capacities roughly
equal to the predicted, maximal adsorption capacities of the
open metal sites. Our neutron diffraction experiments
clearly reveal that the primary CH4 adsorption
occurs right on the open metals. DFT calculations show that
the binding energies of CH4 on the open metal
sites are significantly higher than those on typical
adsorption sites in classical MOFs.
H.
Wu,
W. Zhou,* and T. Yildirim,
J. Am. Chem. Soc., 131, 4995 (2009).

Using neutron powder diffraction, we have directly
determined the methane sorption sites in two prototypical
MOF materials: ZIF-8 and MOF-5. The primary methane
adsorption sites are associated with the organic linkers
in ZIF-8 and the metal oxide clusters in MOF-5. Methane
molecules on these primary sites possess well-defined
orientations, implying relatively strong binding with the
framework. With higher methane loading, extra methane
molecules populate the secondary sites and are confined in
the framework. The confined methanes are orientationally
disordered and stabilized by the intermolecular
interactions. An unusual reversible methane-induced
structural phase transition in MOF host lattice is
observed at ~60 K in both ZIF-8 and MOF-5 due to strong
intermolecular interaction between confined methane
molecules in the pores of the host lattice.
H. Wu,* W. Zhou and T. Yildirim,
J. Phys. Chem. C,
113, 3029 (2009).

11/2008
Methyl groups in ZIF-8 exhibits interesting quantum
rotation behaviorThe reorientational motion of the
methyl group is an intriguing physics phenomenon. It can
be well described by classical random jumps at high
temperature, whereas at low temperature it is dominated by
quantum-mechanical rotational tunneling. Using neutron inelastic
scattering and diffraction, we have studied the quantum
methyl rotation in ZIF-8. The rotational potential for the
CH3 groups in ZIF-8 is shown to be primarily
3-fold in character. The ground-state tunneling
transitions at 1.4 K of 334 ± 1 μeV for CH3
groups in hydrogenated ZIF-8 and 33 ± 1 ueV for
CD3 groups in deuterated ZIF-8 indicate that the barrier to internal rotation is small
compared to almost all methylated compounds in the solid
state and that methyl-methyl coupling is negligible.
W. Zhou,*
H. Wu, T.
J. Udovic,* J. J. Rush, and T. Yildirim,
J. Phys. Chem. A, 112, 12602
(2008).

10/2008
H2 binding strength found to depend strongly on open metal
species in MOFs

MOFs with open metal sites exhibit much stronger
H2 binding strength than classical MOFs.
Yet, how the binding strength varies
with different open metal species was previously unknown. We conducted a
systematic study of the H2 adsorption on a series of
isostructural MOFs, M2(dhtp) (M=Mg, Mn, Co, Ni, Zn). The
experimental Qst for H2 of these MOFs
range from 8.5 to 12.9 KJ/mol, with increasing Qst in the following order: Zn, Mn, Mg, Co, and Ni. The H2 binding energies derived from DFT calculations follow the same
trend. We also found a strong correlation between the metal ion
radius, the M-H2 distance and the H2 binding
strength, which provides a viable, empirical method to predict the
relative H2 binding strength of different open metals.W. Zhou,* H. Wu,
and T. Yildirim, J. Am. Chem. Soc.,
130, 15268
(2008).

08/2008
Origin of the exceptional negative thermal-expansion in
MOF-5 identifiedMetal-organic framework-5 possesses an
exceptionally large negative thermal-expansion (NTE)
coefficient. Our direct experimental measurement, in the
temperature range of 4 to 600 K, shows that the linear
thermal-expansion coefficient is ~ -16×10-6 K-1. From first-principles lattice
dynamics calculations, we deciphered the origin of this large NTE
behavior. We found that almost all low-frequency lattice vibrational
modes (below ~23 meV) involve the motion of the benzene rings and
the ZnO4 tetrahedra as rigid units and the carboxyl groups
as bridges. These rigid-unit modes exhibit various degrees of
phonon softening and thus are directly responsible for the large
negative thermal expansion in MOF-5.
W. Zhou,* H. Wu, T. Yildirim, J. R. Simpson, and A. R. Hight Walker, Phys. Rev. B, 78, 054114 (2008).

04/2008
Nature of open metal-H2 interaction in MOFs
elucidated It's well-known in coordination
chemistry that transition metal-H2 interactions
are often of so-called "Kubas-type". MOF compounds with
exposed transition-metal sites exhibit impressive heats of adsorption
of H2, thus the open metal-H2 interaction was
also widely believed
to be of "Kubas-type". Our first-principles calculation
clearly shows that the H2 binding on the open
metal site in Mn4Cl-MOF (a representative MOF
compound with open metal sites) is not of the expected Kubas-type.
Instead, the major contribution to the overall binding comes from the
classical Coulomb interaction, which is not screened due to the
open-metal site. We also show that the orientation of H2 has a surprisingly large effect on the binding potential, reducing
the classical binding energy by almost 30%.W. Zhou,* and T. Yildirim,
J. Phys. Chem. C, 112, 8132 (2008).

10/2007
High-quality H2 and CH4 adsorption
isotherm data reported for MOF-5 and ZIF-8We
reported H2 and CH4 adsorption
isotherms in two prototypical MOF compounds (MOF-5 and
ZIF-8) over a large temperature (30-300 K) and pressure
(up to 65 bar) range using a fully computer-controlled
Sieverts apparatus. At low temperatures, the maximal
excess adsorption capacities of H2 and CH4
in MOF-5 are found to be 10.3 wt % and 51.7 wt %,
respectively, while they are only 4.4 wt % and 22.4 wt %
in ZIF-8. From the temperature-dependent isotherm data,
the isosteric heat of adsorption (Qst)
was also estimated. The excess Qst’s for
the initial H2 and CH4 adsorption in MOF-5 are
~4.8 kJ/mol and ~12.2 kJ/mol, respectively. We obtained
similar Qst’s for ZIF-8. The detailed
isotherm curves reported here over a large temperature and
pressure range will be a critical test for future grand
canonical Monte Carlo simulations and force-field models.
[download the isotherm data in
this paper]W. Zhou,
H. Wu, M. R. Hartman, and T. Yildirim,*
J. Phys. Chem. C, 111,
16131 (2007).

03/2007
Neutron diffraction study reveals where H2 is
stored in prototypical ZIF-8 compoundZeolitic Imidazolate Frameworks (ZIFs), as a
new subfamily of metal-organic framework compounds, possess the
exceptional chemical stability and rich structural diversity of
zeolites, and show
great promise for H2 storage. Using the difference-Fourier analysis of neutron
powder diffraction data along with first-principles calculations, we
successfully revealed the H2 adsorption sites and the H2 binding energies within
the nanopore structure of ZIF-8. Surprisingly, the two
strongest adsorption sites are both directly associated
with the organic linkers, instead of the ZnN4
clusters, in strong contrast to classical MOFs, where the
metal-oxide clusters are the primary adsorption sites.
These observations are important and hold the key to
optimizing this new class of ZIF materials for practical H2
storage applications.
H.
Wu, W. Zhou, and T. Yildirim,*
J. Am. Chem. Soc.,
129, 5314 (2007).

11/2006
Mechanical properties and lattice dynamics of MOF-5
studied in detail
By combining neutron inelastic scattering (NIS) and
first-principles calculations, we investigated the lattice
dynamics of MOF-5. The structural stability of MOF-5 was
evaluated by calculating the three cubic elastic constants.
We find that the shear modulus, c44=1.16 GPa,
is unusually small, while two other moduli are relatively
large (i.e., c11=29.42 GPa and c12=12.56
GPa). The phonon dispersion curves and phonon density of states
were directly calculated and our simulated NIS spectrum
agrees very well with our experimental data. Several
interesting phonon modes are discussed, including the
softest twisting modes of the organic linker.
W. Zhou* and T.
Yildirim, Rhys. Rev. B, 74, 180301(R) (2006).